1. Field of the Invention
This invention relates to a process for forming a semiconductor thin film by plasma CVD (chemical vapor deposition). More particularly, this invention relates to a process for forming an amorphous semiconductor for making up semiconductor functional devices such as solar cells, as exemplified by a process for forming by plasma CVD a photovoltaic device such as a solar cell making use of amorphous silicon or amorphous silicon alloy. This invention also relates to an amorphous-silicon solar-cell device obtained by such a process.
2. Related Background Art
Plasma CVD is a process commonly used as a process for forming amorphous semiconductor thin films and microcrystalline semiconductor thin films.
The plasma CVD is a process in which material gases are fed into a chamber and the chamber is evacuated by means of an evacuation pump, where direct-current electric power or high-frequency or microwave electric power is applied to cause the material gases to ionize, dissociate and excite in the form of plasma to form a deposited film on a substrate. Conventionally, glow discharge or RF (radio frequency) discharge making use of high-frequency power has been employed, using parallel-plate electrodes.
Where silicon-type amorphous or microcrystalline semiconductor thin films are formed, used as material gases are, e.g., SiH4, Si2H6, SiF4 and Si2F6. BF3, B2H6, PH3 and so forth are also used as dopant gases. Also, where silicon-germanium amorphous thin film or microcrystalline semiconductor thin film is formed, GeH4 gas is often used as a material gas in addition to the above gases. Pressure (for plasma) in the chamber ranges from about 13 Pa to about 1,330 Pa when electric power of from direct current to high-frequency power is applied. When microwave power is applied, the pressure ranges from about 0.13 Pa to about 133 Pa. The substrate is heated to a temperature of from 200 to 400xc2x0 C.
Here, an example is described in which amorphous silicon thin films are formed by commonly available plasma CVD making use of a plasma CVD reactor which is a typical semiconductor thin-film formation apparatus, as shown in FIG. 2. In FIG. 2, reference numeral 1 denotes a deposited-film-forming chamber; 2, an evacuation pump (rotary pump and mechanical booster pump); 3, an evacuation piping; 4, a valve; 5, a conductance regulation valve: and 6, a controller of the conductance regulation valve. Reference numeral 7 denotes the cathode electrode: 8, a high-frequency power source; 9, a matching assembly; 10, a high-frequency power guide; 11, a substrate holder: 12, a substrate; 13, a heater; 14, heater controller; and 15, a heater power source. Reference numeral 16 denotes gas cylinders; 17, a gas flow rate controller; and 18, a gas feed pipe. Reference numeral 19 denotes a pressure gauge; and 20, a discharge (plasma) space.
The substrate 12 is fixed to the substrate holder 11, and a substrate take-in/out opening (not shown) is closed, where its inside is evacuated by means of the evacuation pump 2 so as to produce a vacuum. The substrate 12 is heated to temperature of conditions for deposited-film formation by means of the heater 13, fixed to the substrate holder 11. Into the discharge space 20 inside the chamber 1, a plurality of material gases for deposited-film formation (SiH4, Si2H6, H2, doping gas) whose flow rates have been controlled by the gas flow rate controller 17 are mixedly fed from the gas cylinders 16 through the gas feed pipe 18. To the cathode electrode 7, a desired high-frequency (13.56 MHz) power is applied from the high-frequency power source 8 to cause discharge to take place in the discharge space 20 formed between the cathode electrode 7 and the substrate 12 and substrate holder 11 which face the cathode to serve as the anode electrode. The discharge is regulated by the matching assembly 9. The gases inside the chamber are driven off through the evacuation piping 3, and are always kept replaced with gases fed anew. The pressure of the discharge space 20 is monitored by the pressure gauge 19. Signals of this pressure are sent to the controller 6 of the conductance regulation valve 5 provided in the evacuation piping 3, which regulates the degree of opening of the conductance regulation valve 5 to control the pressure inside the discharge space 20 to a constant level. The material gases for deposited-film formation are ionized, dissociated and excited in the plasma inside the discharge space 20 to form a deposited film on the substrate.
The conductance regulation valve 5 is useful for regulating the pressure to any desired level without dependence on the flow rates of material gases. The conductance regulation valve 5 changes the cross-sectional area of the evacuation piping 3 to increase or decrease evacuation conductance.
After the deposited-film formation is completed, the feeding of material gases is stopped, and a purging gas (He, Ar or the like) is newly fed in to displace the material gases remaining in the deposited-film-forming chamber 1 and evacuation pump 2. After purged, the deposited-film-forming chamber 1 is left to cool, where its inside is returned to atmospheric pressure and the substrate with the deposited film formed thereon is taken out.
Where i-type semiconductor layers of p-i-n semiconductor devices are formed by the above process, as disclosed in U.S. Pat. No. 5,034.333, it is reported that highly efficient solar cells can be obtained by forming the i-type semiconductor layer at a low-controlled high-frequency power at its part vicinal to the p- or n-type semiconductor layer. As also disclosed in Japanese Patent Publication No. 7-99776 and Japanese Patent Application Laid-Open No. 6-85291, it is reported that solar cells can be improved in characteristics by forming a buffer layer at a low rate of film formation in the vicinity of the p- or n-type semiconductor layer. As an easy method for lowering such a rate of film formation, there is a method in which RF power is lowered.
However, as a result of extensive studies repeatedly made in order to form high-quality semiconductor thin films by plasma CVD, it has become clear that, where in the p-i-n semiconductor devices the i-type semiconductor layer is formed for its part vicinal to the p- or n-type semiconductor layer as in the above prior art, the characteristics are not necessarily be improved even when the rate of film formation is lowered or, more directly, the high-frequency power applied to the discharge furnace is lowered to form the buffer layer at the p/i or n/i interface.
Accordingly, an object of the present invention is to provide a process for forming a high-quality semiconductor thin film by plasma CVD and by a new method different from the above prior art, and an amorphous-silicon solar-cell device obtained using such a process.
The present invention is characterized by constituting a semiconductor thin-film formation process as described below, in order to solve the above problems.
That is, the semiconductor thin-film formation process of the present invention is characterized by a semiconductor thin-film formation process comprising feeding a material gas for a semiconductor thin film into a discharge space formed in an inside-evacuatable film-forming chamber, and applying a high-frequency power thereto to cause plasma to take place and decompose the material gas to form an amorphous semiconductor thin film on a desired substrate, wherein,
the high-frequency power is applied changing its power density continuously or stepwise from a high power density to a low power density and thereafter again changing the power density continuously or stepwise from a low power density to a high power density, to form a semiconductor thin film made different in film quality in the direction of layer thickness while retaining the same conductivity type.
The semiconductor thin-film formation process of the present invention is also characterized in that the high power density and the low power density may be in a ratio extending at least twice.
The semiconductor thin-film formation process of the present invention is also characterized in that the material gas may comprise a material gas containing at least silicon.
The semiconductor thin-film formation process of the present invention is also characterized in that the semiconductor thin film may be an amorphous silicon film, and the high power density and low power density of the high-frequency power to be applied are power densities such that the amorphous silicon film at its part formed at the corresponding power density shows Raman shift having peaks positioned within the range of from 490 cmxe2x88x921 to 510 cmxe2x88x921 and the range of from 470 cmxe2x88x921 to 490 cmxe2x88x921, respectively.
The amorphous-silicon solar-cell device of the present invention is also characterized by comprising a p-i-n amorphous-silicon solar-cell device, wherein the Raman shift at an i-type amorphous silicon layer at its part vicinal to the n/i interface or vicinal to the p/i interface has a peak positioned within the range of from 490 cmxe2x88x921 to 510 cmxe2x88x921 and the Raman shift at the other part of the i-type amorphous silicon layer has a peak positioned within the range of from 470 cmxe2x88x921 to 490 cmxe2x88x921.